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Question

# The index of refraction is always larger than or equal to 1, but why is that?

Solution

Verified

Index of refraction of certain medium is defined as a ratio of speed of light in vacuum $c$ and speed of light in that medium $\upsilon$:

$n=\dfrac{c}{\upsilon}$

Since the speed of light in any material medium is lower than the speed of light in vacuum $c$, denominator in the right part of the equation above ($\upsilon$) will always be lower than the numerator in the right part of the equation above ($c$). This means that index of refraction $n$ will always be greater of equal to $1$.

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